Quantification of Sunitinib and N-Desethyl Sunitinib in Human EDTA Plasma by Liquid Chromatography Coupled With Electrospray Ionization Tandem Mass Spectrometry: Validation and Application in Routine Therapeutic Drug Monitoring

Background: Given the low therapeutic index, the large interindividual variability in systemic exposure and the positive exposure-efficacy relationship of sunitinib, there is a rationale for therapeutic drug monitoring (TDM) of sunitinib. To support TDM, a method for determination of sunitinib and its active metabolite (N-desethyl sunitinib) has been developed and validated. Methods: For determination of sunitinib and N-desethyl sunitinib in human EDTA plasma samples, high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) was used. Validation experiments according to Food and Drug Administration guidelines were performed. In addition, the results of 25 analytical runs with 58 patient samples using 8 calibrators and 3 levels of quality control (QC) samples per analysis were compared with the results of analyses using only 3 calibrators and 1 QC sample to accelerate sample turnaround time. The method comparison experiment was performed according to international guidelines. Results: The HPLC-MS/MS method was validated over a linear range from 2.5 to 500 ng/mL using 50 mu L plasma volumes, with good intra-and interassay accuracy and precision. In addition, the mean of the absolute differences between the compared methods was only -0.66 ng/mL (mean of relative differences, -0.85%), which is not a clinically relevant difference. Conclusions: This method has been applied successfully for routine TDM purposes for patients treated with sunitinib. Moreover, reliable results with a rapid turnaround time were obtained when performing a short analytical run containing only 3 calibrators and 1 QC sample.